Finding out identities of user terminals, such as mobile phones, of different users has become increasingly difficult. Conventionally, personal user information is given when the users are registered as telephone subscribers, and the identifications of subscriptions and user equipment of a specific user can easily be established by using the information received in the registration process. However, lately so-called prepaid accounts enabling registering as subscribers without submitting any personal user information have become increasingly common. The prepaid accounts are also often used for criminal purposes when there is a motivation to hide the used equipment and accounts in such a way that for example legal monitoring or tracing of calls by a public authority becomes impossible.
Mobile phone can also easily fall into wrong hands due to theft or careless mistakes. Further, illegal copying of SIM (subscriber identity module) cards of mobile phones has become common. This enables unauthorized use of the same equipment identifiers that are used by a user terminal of the actual, legal user, and at the expense of the legal user.
Because of the foregoing reasons, problems more increasingly occur in finding out the identifiers of user equipment used by suspicious persons, for example, when starting of legal monitoring or supervision by a public authority is needed. A police authority may, for example, know personal information with an address of a suspicious person; however, finding out the identifiers of the mobile phones used by this person may not anymore be possible.
Further, reliable positioning systems are needed particularly in emergency situations, when operating in difficult circumstances and in different rescue operations, for example to locate missing persons through their mobile stations. Positioning should also be possible for example in a situation where the user of a mobile station is not personally capable of using the device. It is also possible that a person in distress succeeds in making an emergency call but does not know his/her location, or the call is disconnected before the person is able to tell the location. A reliable positioning system is also needed when a person on a hiking tour, for example, is unable to call for help because he/she is in an area that is not covered by the public radio network.
In some radio systems, entering of user terminals and the identifications thereof to an unauthorized network, not being a part of the network of a public operator, has been inhibited. For example, in 3G (3rd generation mobile communications) systems, there is a procedure called an Integrity Protection that enables the user terminals to verify that they are connected to the actual 3G network. This is done by appending a message authentication code calculated over the sent messages and checking the authentication code at the receiving end. Message authentication code is calculated by using a secret key of a user terminal that is only known by the user terminal itself and by an authorized network.
Integrity protection is used to protect sensitive messages exchanged between user terminals and the radio network controller, preventing user terminals from continuing communication for e.g. up to the call setup phase. In the case integrity protection is not turned on by the network after radio resource control connection setup, the user terminal will release the connection. This makes radio direction finding using a method described in a patent application WO2006/048508 of the applicant unfeasible without the access to the user terminals' secret keys. WO2006/048508 describes a method for positioning a mobile station. However, the method described in this application cannot be used e.g. when these secret keys of user terminals are not available because of the integrity protection measures by a 3G network, for example.